The present invention relates to a semiconductor memory device having a plurality of non-volatile memory transistors of electrically writable and erasable by ultra-violet rays (hereinafter called as UV-PROM), and more particularly to a memory device in which a redundancy memory cell array is incorporated.
UV-PROM's are widely used in a semiconductor memory device as described in U.S. Pat. No. 4,527,259 and U.S. Pat. No. 4,727,221. On the other hand, a semiconductor memory device having a redundancy memory cell array is shown, for example, in U.S. Pat. No. 4,281,398 and U.S. Pat. No. 4,428,068, in which a main memory cell section, a redundancy memory cell section and a redundancy control section for deciding whether the redundancy memory cell section is to be used or not with respect to received address signals, are provided.
As a program means in the redundancy control section by which an information corresponding to a defect line produced in the main memory cell section is set, polysilicon fuses, UV-PROM's, EE-PROM's (electrically erasable PROM), polysilicon layers having a high electrical resistivity, etc. are employed. When UV-PROM's are used as memory cells in the main memory cell section and in the redundancy memory cell section, UV-PROM's are beneficially used in the redundancy control section as the program means in view of the high reliability of the programming and the easiness of manufacturing because they can be formed with the memory cells in the memory sections. However, the UV-PROM's in the redundancy control section must be shielded from ultra-violet, rays so that the programmed information stored in the floating gate electrodes in the UV-PROM's in the redundancy control section is not eliminated during erasing operations for the UV-PROM's as the memory cell's in the memory cell sections. Therefore, in the prior art, the UV-PROM in the redundancy control section is covered by a metal layer. However, the metal layer can not be provided at a section where the control gate electrode is led out, and an aperture of the metal layer is inevitably formed there. Consequently, some ultra-violet rays are introduced into the metal cover structure through the aperture, and unfavorably eliminate the information, that is, unfavorably release electrons stored in the floating gate electrode.